A wide variety of vertically movable doors are currently used in industrial and other settings. For example, one type of vertically movable door comprises a conventional sectional door having of a series of hingedly attached panels with rollers disposed on opposed lateral edges. Such sectional doors are typically movable between a closed or blocking position and an open or storing position relative to a doorway. For this purpose, a curved guide track--having a substantially vertical portion disposed adjacent to the doorway opening, a substantially horizontal portion disposed above and behind the doorway, and an intermediate curved portion--is provided on either side of the doorway for receiving the rollers of the panels. In this way, the door is substantially horizontal when in the storing position. Another type of vertically movable door is a vertically-storable door which moves in a continuous plane between the blocking and storing positions along substantially straight and vertical guide tracks.
Still other types of vertically movable doors are also available. For example, slat-style doors include a plurality of hingedly attached metal strips or extension members which engage and move along guide tracks arranged on either side of the doorway. In the stored position, these metal strips are rolled-up onto a tube which is disposed above the doorway. Fabric-style doors are also available. Such doors typically include a rigid bottom bar or extension member disposed along the bottom edge of a fabric panel which engages guide tracks arranged on either side of the doorway. In operation, these tracks guide the bottom bar as the door moves between the blocking and stored positions.
With most vertically movable doors, a counterbalancing device is provided for counteracting the effect of gravity on the door as it move between the blocking and storing positions. Such counterbalancing devices typically include a cable system which is attached to the door and a spring which is operatively connected to the cable system. When the door moves from the storing position, the spring becomes stressed and provides a force which counteracts the weight of the door. In this way, the spring controls the descent of the door as it moves towards the doorway-blocking position. Conversely, when the door is raised from the doorway-blocking position, the spring provides a lifting force which makes the door easier to raise.
If the counterbalancing mechanism fails and/or the cable losses tension or becomes slack, however, the spring becomes inoperative as a counterbalance and the door is no longer prevented from free-falling from the storing position. For this reason, some vertically movable doors are provided with a safety mechanism which halts the downward movement of the door should the cable break or should some other failure cause a loss of cable tension. Such safety mechanisms, however, suffer from some noted disadvantages including, for example, general unreliability, fragility, excessive cost, and inconvenient or awkward operation.